{ diff(x1, t) = (1*k4*k1*x1 + (-1)*k4*x1*x3)/k4,
  diff(x2, t) = (1*k4*x1*x3 + (-1)*k4*k5*x2)/k4,
  diff(x3, t) = (1*k4*k5*x2 + (-1)*k4*(k3*x1*x3 + k2*x3))/k4 }